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Preliminary Flood Risk Analysis Report

Waterways Ireland

18th July 2011

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Table of Contents

Executive Summary 3

1.0 Background and Introduction 4

2.0 Waterways Ireland 5

Figure 1 Waterways of Ireland 6

3.0 Flooding Mechanisms 7

4.0 Grand Canal 8

5.0 Royal Canal 12

6.0 Lough Allen Canal 15

7.0 Jamestown Canal 16

8.0 River Blackwater / Errina-Plassey Canal 17

Appendix 1 Grand Canal 21

Appendix 2 Royal Canal 22

Appendix 3 L. Allen & Jamestown Canals 23

Appendix 4 L. Allen Canal Map 24

Appendix 5 Jamestown Canal Map 25

Appendix 6 R. Blackwater Embankments 26

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Executive Summary

The statutory function of Waterways Ireland, the largest of the six North/South Implementation

Bodies established under the British-Irish Agreement Act 1999, is to manage, maintain, develop

and restore specified inland navigable waterways; the Barrow Navigation, the Lower Bann

Navigation, the Royal Canal, the Erne System, the Shannon-Erne Waterway, the Grand Canal

and the Shannon Navigation principally for recreational purposes.

The Statutory instrument transposing EU ‘Floods’ Directive into Irish law identifies roles for

organisations such as local authorities, Waterways Ireland and ESB to undertake certain duties

with respect to flood risk within their area of responsibility. Such risks must be identified

through a preliminary flood risk assessment by December 2011. The PFRA is a high level

screening exercise which involves collecting existing and readily available information on

historic and potential floods, assembling it into a preliminary assessment report and using it to

identify Flood Risk Areas which are areas where the risk of flooding is significant.

This report looks at the possible flooding mechanisms arising from the ‘artificial water bearing

infrastructure’ and includes an analysis of historic flooding and potential future flooding of the

Grand and Royal Canals and other smaller canals linked to the Shannon Navigation, the Lough

Allen Canal, the Jamestown Canal and the River Blackwater / Erina-Plassey Canal.

Conclusion

The analysis of historic data shows that, while there have been incidences of flooding caused by

failure of embankments and operational issues on the Grand and Royal Canals, they have

generally occurred in rural areas with very limited damage to property. In only 2 cases a small

number of houses and businesses were affected but for the remainder of cases the damage has

been limited to temporary flooding of bog or farmland. In Tullamore and Edenderry the ground

levels are lower than the canal in some areas and there is a potential for some flooding of

property but the only area where the consequences of an embankment failure is relatively high

is the embanked section of canal close to Mullingar, Co. Westmeath where up to 200 houses

could be flooded. However this embankment has no history of failure, has been strengthened

and partially lined in recent years, is inspected weekly for any sign of a potential breach and

remedial action would be put in place immediately so while the consequences would be

significant the likelihood of failure is extremely low and therefore this is not considered to be

an area of significant flood risk.

Waterways Ireland is committed to continuing to work with the Office of Public Works and the ESB to deliver the Assessment and Management of Flood Risks on designated waterways as required by EC Dir 2007/60/EC.

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1.0 Background and Introduction

Between 1998 and 2004 Europe suffered over 100 major damaging floods including the

catastrophic floods along the Danube and Elbe rivers in Summer 2002. Further severe

floods in 2005 further reinforced the need for a co-ordinated approach to the

management of the problem. Since 1998 floods in Europe have caused up to 700 deaths,

the displacement of 500,000 people and at least €25 billion in insured economic losses.

Catastrophic floods endanger human lives and cause human tragedy as well as heavy

economic losses and can have severe environmental consequences. Floods are natural

phenomena but through the right measures it is possible to reduce their likelihood and

lessen their impact.

Directive 2007/60/EC on the assessment and management of flood risks aims to reduce

and manage the risks that floods pose to human health, the environment, cultural

heritage and economic activity. Under S.I. 122 of 2010 European Communities

(Assessment and Management of Flood Risks) Regulations 2010, the Statutory

Instrument transposing the EU Directive into Irish Law, the Commissioners of Public

Works in Ireland are appointed as the Competent Authority for flood risk management

and other local authorities and organizations are named. Waterways Ireland, as the

statutory body responsible for the majority of Ireland’s inland navigable waterways, is

obliged to undertake tasks the first of which is to prepare a Preliminary Flood Risk

Assessment (PFRA) of the potential flood risk posed by the structural or operational

failure of any of its infrastructure.

The PFRA is a high level screening exercise which involves collecting existing and readily

available information on historic and potential floods, assembling it into a preliminary

assessment report and using it to identify Flood Risk Areas which are areas where the

risk of flooding is significant. This PFRA concentrates on flooding which has arisen or is

likely to arise from the Royal and Grand Canals, classified in the legislation as ‘artificial

water bearing infrastructure’.

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2.0 Waterways Ireland

Waterways Ireland is the largest of the six North/South Implementation Bodies which

was established by means of an international treaty made on 8 March 1999 between

the British and Irish Governments. This treaty was given domestic effect by means of

the North/South Co-operation (Implementation Bodies) (Northern Ireland) Order 1999,

and the British-Irish Agreement Act 1999 respectively.

As a Cross Border body, Waterways Ireland operates under the policy direction of the

North / South Ministerial Council and the two Governments, and is accountable to the

Northern Ireland Assembly and the Houses of the Oireachtas.

The statutory function of Waterways Ireland is to manage, maintain, develop and

restore specified inland navigable waterways, principally for recreational purposes.

Waterways Ireland has responsibility for approximately 1,000 km of navigable

waterways (Figure 1) comprising;

the Barrow Navigation

the Lower Bann Navigation

the Royal Canal

the Erne System

the Shannon-Erne Waterway

the Grand Canal

the Shannon Navigation

Waterways Ireland’s remit was extended by the North South Ministerial Council in July

2007 to include responsibility for the reconstruction of the Ulster Canal from Upper

Lough Erne to Clones and following restoration for its management, maintenance and

development principally for recreational purposes.

Of the water bodies listed above the artificial water bodies are the Grand Canal, the

Royal Canal, part of the Shannon-Erne Waterway and a number of smaller canals linked

to the Shannon Navigation namely the Lough Allen Canal, the Jamestown Canal and the

Erina Plassey canal. The other navigation systems are a mix of River/Lake navigation

with short lateral canals. Flooding on these systems is being dealt with under the fluvial

PFRA being prepared by the Office of Public Works.

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Figure 1

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3.0 Potential Flooding Mechanisms

The possible flooding mechanisms arising from canal infrastructure are:

3.1 Failure or Breach of an Embankment

A large proportion of the Grand and Royal Canals are built in embanked sections running

at a higher level than the surrounding countryside. These embankments were

constructed of local readily available material, sometimes stone and clay but in some

cases they are soft peat embankments which require considerable maintenance. Failure

or breach of these embankments results in water from the level being released but the

impact of the flood waters very much depends on the time of year and the level of

saturation of the surrounding area. The tables in Appendix 1 & 2 shows the maximum

volume of water which would be released by a failure of each of the levels of the Grand

and Royal Canals.

3.2 Overtopping of the Banks

During periods of intense or prolonged heavy rainfall the volume of water running into

the canal can exceed the volume of water which can be racked off using the overflows,

the land tunnels and the gate sluices. This excess water overtops the banks and can

cause flooding of surrounding areas if it cannot be discharged through the drainage

network. The primary risk to the canal system of water entering at a rate which cannot

be discharged or managed is that the canal water levels rise and will overtop. In

embanked areas there is then a risk of failure particularly due to the erosion of the top

bank level.

3.3 Operational Issues

Water has to be managed through the canal system to keep all levels at their optimum

depth and sluices in the gates are used to carefully monitor the amount of water flowing

from one level to the next. Overtopping from a long level to a shorter level can result in

the shorter level being unable to discharge the volume of water and resultant flooding

of the surrounding areas. Any failure of the lock-gates or interference with the sluices

whether deliberate through acts of vandalism or accidental can result in overtopping of

a short level as described above.

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4.0 The Grand Canal

4.1 History of Construction

Work on the Grand Canal from Ringsend to the River Shannon, crossing the central plain

and the Bog of Allen, commenced in 1756 and was complete to the Shannon in 1804.

The canal is 182km long including the Branch Lines to Naas and Edenderry. The summit

level at Lowtown is 40km west of Dublin and 85m above low tide at Dublin where there

are 3 sea locks linking the Grand Canal Basin with the tidal River Liffey. The rise from

Dublin to the summit is by way of 26 locks and the 50m fall to the River Shannon is by

way of 18 locks over a distance of 93km. The average rise or fall of the locks is 3.0m

while the largest is 5.7m at Inchicore Lock.

The Barrow Line of the Grand Canal is 45km from the summit at Lowtown to where it

joins the Barrow River at Athy. The descent to the Barrow is by way of 9 locks, 2 of

which are double-chambered.

There are 14 supply channels feeding the system at various lengths totally

approximately 64km; the principal one being the Milltown supply from Pollardstown Fen

which feeds the summit level. Most of the supply channels are artificially constructed

and require constant maintenance particularly where they are embanked or through

bog sections. From the summit level at Lowtown the canal begins its slow descent to

Shannon Harbour where it joins the River Shannon. It passes through a varied landscape

a particular feature of which is the high embankments with 24km through bogs.

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4.2 Historic Flooding on the Grand Canal

4.2.1 Flooding due to embankment failure

Approximately 50% of the Grand Canal is built in embanked sections at a higher level

than the surrounding countryside. These embankments were constructed in the late

1700s of readily available local material sometimes stone and clay but 24km are soft

peat embankments which require considerable maintenance. There were numerous

breaches of these embankments during the 1800s but none resulted in any significant

flooding or damage to property, mainly due to the rural nature of the surrounding

countryside much of which is bog.

The most significant breach of a peat embankment was the Edenderry breach in 1989

which occurred in a 31.5km level, the longest level of the Grand Canal. The Edenderry

embankment, stretching from the Blundell Aqueduct to Downshire Bridge, is

constructed entirely from turf, the only material available to the builders in the 18th

century. When this section was first watered in 1797 it promptly collapsed and this was

followed by further failures in 1800, 1855 and 1916. The most recent failure occurred in

January 1989 when a large breach opened in the North bank about 950m west of the

Blundell Aqueduct near the town of Edenderry. It is estimated that up to 135,000 m3 of

water was discharged through the breach and 100,000 m3 of embankment material was

displaced over a length of 300m; however, the damage to the surrounding land was

comparatively minor. Approximately 12 acres of land were flooded with short term

flooding of a further 36 acres including some football fields. This receded within a day

and left little or no residual damage.

The failure was the result of a long length having become saturated over a period of

many years. When the canals were commercially used 12 boats were continuously

employed claying this section to avoid leaks. However, this ceased when the canals were

closed to commercial traffic resulting in the peat becoming completely saturated and

the continuing leaks causing piezometric pressure to build up near the base of the

embankment. Long term wetting deteriorates the strength of the peat and eventually a

point was reached, in this case, when the resistance due to the shear strength of the

peat became less than the upward piezometric pressure resulting in a large wedge of

the embankment simply floating away.

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Similar breaches occurred in the Derries Embankment in 1955 and the Killeen

embankment in 1975. In the former case the water was discharged to the Silver River in

the immediate vicinity and the latter resulted in the flooding of some bogland. In August

1993 the partial collapse of a culvert at Hartley Bridge, Ticknevin, Co. Kildare resulted in

the loss of approximately 30,000m3 of water. This did not result in any damage to land

but did cause some inconvenience to 3 dwellings during the repair of the culvert.

4.2.2 Flooding due to overtopping and operational issues

There have been a number of minor flooding incidents caused by overtopping and

operational issues.

In the Bluebell area of Dublin city in November 2005 some damage was caused

to 5 business premises due to vandalism at locks which resulted in bank

overtopping.

Some flooding occurred in Ardclough village near Celbridge, Co. Kildare in winter

2009 partly as a result of overtopping of the canal bank but no houses or

businesses were affected.

In Ballycommon, Co. Offaly in August 2008 during a period of intense heavy

localised rainfall the large diameter pipes under the towpath could not take the

flow from the surrounding high ground and the water entered the canal flowing

over the towpath. The volume of water entering the canal exceeded the volume

of water which was being discharged via the sluices, overflows and racks. At the

time there was a significant risk of overtopping and emergency services were put

on alert but no damage occurred.

4.3 Inspection and Maintenance Regime

When a breach occurs dams are installed immediately to reduce the loss of water and

the embankment is repaired. In the case of the 1989 Edenderry breach the section of

embankment was completely rebuilt and lined and, as a result of the lessons learned,

peat embankments are now inspected regularly for any signs of damage or leaks and

there is an ongoing program to strengthen them and line them where necessary to

reduce the risk. In addition a stop chamber was constructed at Rathmore which reduces

the length of the level likely to be affected by another breach of this embankment to

8km and so the amount of water which would be discharged would be 25% less than

was discharged in 1989.

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Water is carefully managed throughout the system and all locks are inspected regularly

to ensure that the water management regime is in order. Where there is a risk or history

of vandalism, locks are placed on the sluices to prevent interference. Where banks have

limited freeboard there is an ongoing program to raise them to increase the carrying

capacity of the canal and there is also an ongoing maintenance program for overflows

and back-drains.

4.4 Potential Future Floods

The majority of the Grand Canal runs through remote rural areas much of which is bog

and so flooding will result in limited damage. The canal passes through a number of

villages and 2 towns Edenderry and Tullamore.

Edenderry town ground levels are below canal water level and the toe of the

embankment has been damaged in the past due to industrial activity particularly in the

Edenderry Business Park. A catastrophic failure of this embankment could have serious

consequences to property due to the volume of water contained in the 31.5km level and

the topography of the area; however, some of the risk has been mitigated by the

introduction of the stop chamber, the works done to the embankment and the regular

inspections of the embankment.

The ground levels in some areas of Tullamore town are also below canal water level. The

embankments here are intact but continuous development alongside the canal including

underground services could impact on the structural stability of the canal

embankments. A breach in the Tullamore area would have serious effects on property

however, continuous monitoring and maintenance regimes are in place and a breach is

thought to be unlikely in this area.

Appendix 1 lists all reaches of the Grand Canal system giving dimensions, embankment

details, inflows and potential flooding volumes.

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5.0 The Royal Canal

5.1 History of Construction

The Royal Canal was the second canal to be built across the country from Dublin to the

River Shannon. Work started in 1790 and the canal reached the Shannon in 1817.

Spencer Dock in Dublin was not complete until 1873. The navigation starts at Spencer

Dock and the canal rises steeply out of the city through a succession of double locks.

From the 10th lock, although still in Dublin, it begins to assume a rural aspect through an

attractive tree lined stretch. It climbs up to a summit level through Mullingar at 94.3m

higher than the entry level at Spencer Dock , then drops down to the River Shannon at

approximately 40m above sea level. The canal is 146km in length with 46 locks 10 of

which are double chambered and there is also a sea lock where the canal joins the River

Liffey in Dublin. Approximately 55% of the Royal canal is embanked with 3 peat

embankments at Cloonbreany, Begnagh and Ballymaclavy and a 3km embankment

running around the town of Mullingar, Co. Westmeath. The Royal Canal was closed to

navigation from 1960 and was only fully reopened in 2010 following a lengthy period of

reconstruction.

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5.2 Historic Flooding on the Royal Canal

5.2.1 Flooding due to embankment failure

The only significant breach of the Royal Canal embankments in recent years occurred in

June 1993 on the 32.4km long level of the Royal Canal near the Longwood Aqueduct at

Ballycooley, Longwood, Co. Meath. The breach was approximately 15m wide and

occurred in a 6m high embankment. The water flowed through the breach into a low-

lying strip of waste land and from there through a culvert under the railway and flooded

a lane and some fields. A large area of land was flooded however the floods receded

within 2 days and the breach did not result in any significant damage. A similar breach

occurred in this area in the 1920s.

5.2.2 Flooding due to overtopping and operational issues

The most significant flooding due to overtopping was in the Spencer Dock area in Dublin

city in 2002 when, due to the very high tide levels, the River Liffey was 0.4m higher than

the level in the Royal Canal. The water flowed back up the Royal Canal and caused

flooding of a maximum of 20 houses and 5 business premises.

Other flood events were extremely minor in nature Maynooth Harbour has occasional

flooding of 1 garden if sluices in the lock gates are not left in the correct position and

there is also occasional flooding of the road east of Darcy's Bridge and near Ferns Lock.

5.3 Remedial Action

Immediate repairs were made to the Longwood embankment which was rebuilt and

sealed with a HDPE liner and puddle clay. The embankment is inspected regularly for

any signs of a further breach.

In Spencer dock a new sea lock and flood protection system was constructed so that

high tides can no longer cause this type of flooding.

5.4 Inspection and Maintenance Regime

All of the embankments in the Royal Canal are inspected regularly. Because of the level

of risk the Mullingar embankments are inspected weekly while the Longwood, Downs

and Ballymaclavy embankments are inspected monthly. Any necessary repairs are

carried out immediately.

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5.5 Potential Future Floods

The only area of potentially significant flood risk identified by this study is Mullingar

where up to 200 houses could be flooded in the event of a failure of the embankment

however

this embankment has no history of failure

has been strengthened and partially lined in recent years

is inspected weekly for any sign of a potential breach

remedial action would be put in place immediately

while the consequences of failure would be significant the likelihood of failure is

extremely low and therefore this is not considered to be an area of significant flood risk.

Appendix 2 lists all reaches of the Royal Canal system giving dimensions, embankment

details, inflows and potential flooding volumes.

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6.0 Lough Allen Canal

6.1 History of Construction

The Lough Allen Canal is approximately 7.4Km long. It was constructed in the early 19th

century to connect the Shannon Navigation at Battlebridge to Lough Allen at

Drumshanbo Bridge through Acres Lake, near Drumshanbo. The canal fell into disuse

after 1933 but was restored and reopened as far as Acres Lake in 1977 and fully

reopened to Lough Allen in 1995.

Datum Levels (Poolbeg)

Ordinary summer Level for Lough Allen is 48.16m OD.

The canal is formed on two levels, (1) Drumshanbo to Acres lake to Drumleague Lock

3.18km at OSL of 49.85m. OD and (2) Drumleague to Battlebridge, a length of 2.67km at

OSL (Ordinary Summer Level) of 46.45m OD

The Ordinary Summer level downstream of Battlebridge lock is 42.98m

6.1.1 Storage capacity

The volume of water stored at Level 1 is 221.7 x 103 cubic metres. The volume of water

stored at Level 2 is 57.6 x 103 cubic metres. In times of low water, the level is maintained

by an intake pumps. In times of heavy rainfall, levels are reduced by gravity flow through

lock gate and land sluices.

6.2 Historic flooding on the Lough Allen Canal

While there is a historical reference to bank failure in 1876, the raised embankment

section of the Lough Allen canal performed satisfactorily in recent flooding events.

6.3 Inspection and Maintenance Regime

Water Levels are managed daily by experienced personnel. Inspection regimes are in

place to carry out regular inspections of the canals and amenities. Maintenance

programmes are in place to address reported defects.

During a flood event water levels are monitored daily and sometimes hourly and water

levels are managed to reduce pressure on the banks.

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6.4 Potential Future Floods

Specifically, in consideration of potential floods arising from Waterways Ireland

infrastructure and not from river flooding, the potential risks are associated with failure

of raised canal banks, failure of lock gates, and potential vandalism, neglect or human

error.

There is 3.18km of raised bank on Level (1) and 2.40km of raised bank on Level (02). This

represents 35% and 45% of the canal banks respectively.

7.0 Jamestown Canal

7.1 History of Construction

The Jamestown Canal is located just south of the village of Jamestown on the

Roscommon side of the Shannon. The canal is approximately 2.7km long. It was

originally constructed in 1754 and upgraded in 1845. As a lateral canal the water level is

determined by the upper region and is the same as the Carrick-on-Shannon to

Jamestown stretch with an OSL of 42.98m OD. 24% of Jamestown canal is raised bank.

7.1.1 Storage capacity

As a lateral canal, the canal level is determined by the River Shannon, and therefore

Jamestown Canal will be incorporated in the River Shannon Assessment.

7.2 Historic Flooding on the Jamestown Canal

The canal does not contribute to flood relief. During the 1999/00 and 2009 floods,

temporary dams were required to reduce pressures on the canal banks.

7.3 Inspection and Maintenance Regime

Water Levels are managed daily by experienced personnel. Inspection regimes are in

place to carry out regular inspections of the canal and amenities. Maintenance

programmes are in place to address reported defects.

Appendix 3 contains summary details and dimensions.

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8.0 Shannon Navigation – River Blackwater

8.1 River Blackwater

The River Blackwater is a small tributary of the River Shannon which joins the latter on

its right bank about 2 miles upstream of Limerick City. The catchment area covers

15,500 acres and is entirely in Co. Clare with the village of Clonlara almost in the centre.

From the southern slopes of the Slieve Barnagh range of hills the several streams which

form the river flow from north to south where it then passes under the Headrace of the

Ardnacrusha generating station west of Clonlara and thence in a south-westerly

direction to join the Shannon. Most of the area is steeply sloped having Knockanuartha

(1017ft) and Knockaphunta (845ft.) on its watershed. The lowlands (about 40ft) are just

10miles from the farthest point on the watershed. Due to the very steep slope of the

catchment the river responds extremely quickly to rainstorms. Rainfall in the upper

reaches discharges into the lowland area in about 2 ½ hrs after the commencement of a

storm resulting in floods of high intensity and short duration causing damage to several

hundred acres of land and at times flooding buildings.

8.2 Errina Plassy Canal

When the Commissioners of Inland Navigation and / or the Limerick Navigation

Company in or about 1770 were making the River Shannon navigable from the head of

the tideway at Limerick to Lough Derg they constructed a 6 ½ mile long lateral canal

between Plassy and Errina just downstream of O’Briensbridge to overcome the falls at

Doonass and Castleconnell. A supply of water from the Canal was taken from the River

Shannon at the upstream end and near Errina. No other water was carried in the Canal.

There were 6 navigation locks to overcome a total lift of about 56 feet. The Canal was

not finally completed by the Directors General of Inland Navigation until about 1812.

Ownership passed to the Limerick Navigation Company in 1829, to the Shannon

Commissioners in 1839 to the Commissioners of Public Works in 1846 and to Waterways

Ireland in 1999.

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The Canal has not been used for navigation since 1930 when the Shannon Hydroelectric

Scheme rendered the waterway unusable as a route to Lough Derg and a new

navigation was incorporated in the Headrace Canal of the generating station.

The route of the Canal passes through the lower catchment area and flood plain of the

River Blackwater. For topographical reasons it was not possible to carry the canal

through the flood plain by aqueduct over the River Blackwater. The latter was diverted a

short distance downstream of Mountcatherine Bridge, and both River and Canal were

turned in a south-westerly direction to discharge into the Shannon at Plassy. Both run

parallel and are separated by the canals right embankment for the entire length of the

diversion. This embankment retained the Canal’s operating water level which in summer

was higher than the diverted river. During flood periods however the river level used to

rise higher than the Canal’s level and can overtop the Canal embankment in extreme

conditions.

As part of the diversion works embankments and a sluice were also constructed along

both banks of the River Blackwater diversions as far as the diversion point which is also

the upstream limit of Waterway Ireland’s jurisdiction for Navigation purposes. Upstream

of the diversion point and on both banks of the River Blackwater these embankments

are continued to higher ground just downstream of Mountcatherine bridge. It is not

known by whom the latter were constructed or where responsibility for their

maintenance rests. Waterways Ireland however carries out repairs from time to time on

the embankments downstream of the diversion point. In 1984 a landowner removed

part of the embankment on the left bank of the River Blackwater just downstream of

Mountcatherine Bridge, leaving a large gap in the flood defence works and contributing

to a large extent to the flooding of the 5th / 6th August 1986.

The purpose of the river embankments was to divert all the upland water upstream of

Mountcatherine Bridge along the new channel and to protect the Canal from flooding.

This conferred valuable immunity from flooding on the low lying lands of the Blackwater

though which the Canal is carried as long as the embankments from Mountcatherine

Bridge to Plassy are not breached or overtopped. Even if they are the extent of flooding

cannot be as great as would be the case had the Canal not been constructed since the

diversion carries the bulk of the Blackwater’s discharge away from the old course. The

new channel is ¾ mile shorter than the old course which was extremely tortuous and

must have been inefficient to cater for any flood.

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No records survive to indicate ancillary works carried out affecting the drainage of the

low lying lands on either side of the Canal in the townlands of Mountcatherin,

Newtown, Springfield and Cappavilla North. In times of flood the River Shannon

backwaters along those drains and can cause flooding. The old course of the River

Blackwater was abandoned but the outfall section from the River Shannon to a point

about 1 mile south-east of Newtown Lock was retained to drain the balance of the

Blackwater catchment not served by the diversion. The lands on either side of the canal

are extremely low lying and of poor quality. Moderate rainfall causes waterlogging or

flooding due to the small gradient and poor condition of the channels.

As the canal level is determined by the River Shannon it will be dealt with as part of the

River Shannon study.

8.3 Past Floods on the River Blackwater

Flooding of 5th/6th August 1986

Exceptional rainfall occurred throughout the Southwest and eastern parts of the country

on the night of 5th/6th August 1986 when there was record rainfall in Counties Kerry,

Cork, parts of Limerick and Dublin. However the storm was less intense when it reached

east Limerick and Clare. The following rainfall fell in a period of 12 hours, Ardnacrusha

40mm, Parteen 46.6mm and Shannon Airport 35.5mm. The Metrological Service stated

that the rainfall could have been as high as 50mm in the Clonlara area and considerably

higher in the upper reaches of the Blackwater.

The discharge in the River Blackwater as a result of the rainstorm appears to have been

exceptionally high. The flood peaked in the early hours of the 6th August and an estimate

of the peak discharge was recorded as 2,500 cusecs.

Part of the discharge flowed directly through a breach in the left bank where a local

landowner had removed a large section of embankment. In doing so one dwelling house

and office was flooded to a depth of 2 and a half foot while another house was flooded

to a depth of 6 inches.

Further downstream there was more flooding on the left bank as a result of damage to

the embankments by cattle but no houses were affected.

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8.4 Remedial Action

The embankments were repaired.

8.5 Inspection Regime

The banks are inspected regularly, once weekly and necessary repairs, removal of trees

and debris carried out. The banks are also checked after high winds and heavy rain.

8.6 Potential Future Floods

If the embankment was breached was breached at the same location again, then there

is the potential for more flooding of dwelling houses. There are now 27 houses at that

location and, depending on their floor levels, some or all of those could be at risk of

flooding. This falls outside the significant flood risk as defined for the PRFA Report.

The Inspection Regime mentioned above is in place and there was no flooding at this

location in the extreme flood event of 2009.